What is the best approximation of "Language is a virus." in Latin? 
Rather than just translating sickness (as morbus, for example), I'd
like to emphasize contagiousness.

Lingua virus est.
Lingua contagio est.


Virus is a perfectly good word in Latin, as is pestilentia, of course,
but as you wish to emphasize the contagious rather than the morbid
sense, then contagious itself is derived from Latin and seems natural.
The Latin word for contagion is contagio, contagionis.

Charlton T. Lewis, Charles Short, A Latin Dictionary
http://www.perseus.tufts.edu/cgi-bin/ptext?doc=Perseus%3Atext%3A1999.04.0059%3Aentry%3D%2310725

Language is a contagion.

language = lingua, linguae
contagion = contagio, contagionis
is = sum, esse, fui, futurus (to be).



lingua, nominative, subject of the sentence.

lingua nom.
linguae gen.
linguae dat.
linguam acc.
lingua abl.

contagio, predicative nominative, in apposition to lingua.

contagio nom.
contagionis gen.
contagioni dat.
contagionem acc.
contagio abl.

est, third person present singular.

sum - I am
es - You are
est - He, She, It is


lingua contagio est.


SEARCH TERM

Searched Perseus for contagion.

hlabadie-ga

I have forgotten essentially all of my high-school level Latin
grammar, so I don't trust my judgment on suggesting the right sentence
for you, but my dusty Cassell's New Latin Dictionary says that several
authors (Caesar, Cicero and Livy) conveyed the meaning of "plague,
infectious disease" with the phrase "pestilenti? conflictati."

Thanks for the favorable rating.

hlabadie-ga

No, no, no... thank you for the help!